Update for whoever finds this thread in future.
Apparently my expansion tank is a different revision and has no physical silica bag inside. So no burst silica bag. Also checked with VCDS for errors but aircon unit had no faults.
I ordered an original Valeo heater matrix from Germany for less than 100€, disassembled the entire central tunnel and replaced it.
No leaks around the AC site, but the old heater matrix was almost 100% blocked. After cleaning it I tried to blow inside but only very little air was coming out. Compared to the new it was day and night.
Reassembled everything, refilled the lost coolant, reset all the errors.
Aircon works again, even hotter than before. After another small trip of 1000 km coolant level seems to be stable. Will keep an eye on it in the next weeks.